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It’s worth mentioning that IE6 and below can go a bit extreme when resizing fonts set as ems.

You can fix this by applying font-size: 100% to html. Don’t know why this works, it just does.

Oh, if you’re working in quirks mode you’ll also need to apply it to thead, tbody and tfoot, beacuse the fix doesn’t inherit (don’t apply it to table unless you want table to inherit font-sizes). You don’t need thead, tbody or tfoot in your tables as tbody is implied.

Obviously, you can’t then apply your own font-sizes to html or tbody etc, else you’ll overwrite the fix, which is why I chose those particular elements as they’re rarely used for font-sizes.